The process of nitrating benzene is old and well known and has been commerically practiced for many years to yield mononitrobenzene used, in turn, in the production of aniline. Conventionally, the manufacture of nitrobenzene comprises the batchwise, stepwise or continuous addition of mixed nitric acid and sulphuric acid to benzene. This nitration reaction may be conducted at temperatures controlled in the range of 60.degree.-70.degree. C. with the removal of the heat of reaction or it may be conducted under adiabatic conditions described by Castner in U.S. Pat. No. 2,256,999, where little or no heat of reaction is removed and in which the heat of reaction is utilized in later stages of spent acid reconcentration. The mixed nitric/sulphuric acid employed by Castner in his adiabatic process comprises a mixture of 75% strength sulphuric acid together with sufficient 63% strength nitric acid such that the mixture has a content of 3% nitric acid. In an improved adiabatic process described by Alexanderson et al in U.S. Pat. No. 4,091,042, wherein the reaction is carried out under superatmospheric pressure, the mixed acid contains 3-8.5% nitric acid, from 58.5-70.0% sulphuric acid and not less than about 25% of water. In both the Castner and Alexanderson et al processes, the mixed acid and a stoichiometric excess of benzene are admixed and reacted together under vigorous agitation at temperatures of 100.degree. C. or greater.
Since the acid phase and the benzene phase are not miscible, the reaction rate and the reaction efficiency between the phases are largely limited by mass transfer; that is, by the ability to expose large interfacial areas of each of the phases to each other. As the interfacial areas are increased, the reaction rate between the phases is enhanced. In conventional nitrobenzene production facilities, these interfacial areas are normally created by reacting the two phases in one or more agitated vessels where high shear forces are applied to the liquids. Alexanderson et al described the use of "vigorous agitation" to disperse the benzene throughout the reaction mixture. In other similar nitration processes, various means have been proposed to bring together the immiscible phases. In the process of Toischer et al (U.S. Pat. No. 3,431,312), a cascade of stirred reaction chambers is used in the nitration of toluene. In the process of Terao et al (U.S. Pat. No. 3,160,669), a compartmentalized, elongated, baffled reaction zone containing a series of agitating blades fixed to a stirrer shaft are provided. In the process of Nilsson (U.S Pat. No. 2,737,522), glycerine is nitrated with mixed acid by injecting a pressurized jet of acid into a venturi-shaped reaction zone to contact a similar jet of glycerine within the zone where intimate mixing is caused to take place. McKinney in U.S. Pat. No. 2,951,866 describes the use of a tubular reaction zone wherein separate streams of polyhydric alcohol and nitrating acid are impinged upon each other to form a turbulent reaction mixture. A similar tubular reactor is described by Stow in U.S. Pat. No. 3,111,538. Gebauer, in U.S. Pat. No. 4,251,455, makes reference to the process of German Patent No. 1,135,876 wherein the nitration of polyhydric alcohols is achieved by impinging the two reactants upon each other. In the Chemical Engineering Handbook (Perry), 6th Edition, a number of methods are proposed to achieve intimate mixing or contact between liquids including, for example, in-line motionless mixers, mechanical agitation, gas agitation, jet mixers, injectors, orifice mixers and nozzle mixers.
None of the aforesaid methods for achieving large interfacial areas of contact between immiscible liquid phases is completely satisfactory nor has any method, other than mechanical agitation, been used commerically to any degree in the manufacture of mononitrobenzene. These methods either suffer from high capital and maintenance costs and high power requirements, as in the case of agitated vessels, or they are difficult to control in terms of optimum reaction efficiency as in the case of impinging streams or jets. There, therefore, remains a need for a benzene nitration process which is economic to construct and operate, which is safe and simple to control and which leads to optimum output of reaction product at least possible cost.